1. Field of the Invention
The present invention relates to a microwave receiving device for receiving a microwave such as a satellite broadcast and, more particularly, to a microwave receiving device having functions of a primary radiator and a low-noise converter and suitable for a compact receiving antenna system.
2. Description of the Related Art
A microwave receiving antenna and, particularly, a receiving antenna for satellite broadcast generally comprises a parabolic reflector, a primary radiator, a low-noise converter, an arm, a mount, and a pole. The arm, the mount, and the pole are used to support the reflector, the radiator, and the converter. The parabolic reflector reflects a microwave and focuses it on the primary radiator arranged at a focal point of the reflector. The primary radiator comprises a horn portion for obtaining a desired primary radiation pattern and a circularly polarized wave generator for converting a circularly polarized wave normally used in satellite broadcast into a linearly polarized wave. The low-noise converter converts, e.g, a 12-GHz broadcast signal into a 1-GHz signal and amplifies the 1-GHz signal.
The primary radiator and the low-noise converter are prepared as separate components in a conventional antenna and are coupled through flanges of the radiator and the converter. In a 12-GHz receiving antenna, a primary radiator has an average length of 10 cm, and a low-noise converter has an average length of 15 cm. With the above arrangement, the total length is as large as 20 to 25 cm. In contrast, parabolic reflectors have been made compact because the opening efficiency of such reflectors and the performance of the low-noise converter used in combination with said reflectors have been improved. Therefore, the diameter of a 12-GHz parabolic reflector is generally 40 to 50 cm at present. In the receiving antenna described above, the total length of the primary radiator and the low-noise converter connected thereto is 1/2 the diameter of the parabolic reflector. For this reason, inertia moments of the primary radiator and the low-noise converter are increased and tend to receive a wind pressure. Therefore, strength of the mounting arm for the primary radiator and the low-noise converter must be increased.
An offset type receiving antenna is very popular because it tends not to be adversely affected by snow. Particularly, in this case, the total length of the primary radiator and the low-noise converter is large as compared with the size of the parabolic reflector, thus causing unbalance in design.
As described above, since the conventional primary radiator and low-noise converter are bulky, achieving the necessary rigid structure of the receiving antenna increases cost. In addition, it is difficult to fit the conventional primary radiator and low-noise converter with a compact parabolic reflector.